1. Field of the Invention
The present invention relates generally to an additional service in a communication system. In particular, the present invention relates to a method for providing a Teletype/Telecommunications Device for the Deaf (TTY/TDD) service using a wireless terminal, and a wireless terminal implementing such a method.
2. Description of the Related Art
Communication technologies, particularly mobile communication technologies, have been developed to provide data service as well as voice service. Communication systems provide additional services to satisfy various user demands. One of the additional services is a TTY/TDD service. The TTY/TDD service is a service by which hearing-impaired people can make/receive a telephone call.
A TTY/TDD device is required for the TTY/TDD service. The TTY/TDD device comprises a keyboard, a modem, and a display unit connected to a wired/wireless phone via the modem. The TTY/TDD device converts voice received from another subscriber to text messages and displays the converted text messages on the display unit, and converts text messages input through the keyboard to voice and transmits the converted voice to the wired/wireless phone. By using this, hearing-impaired people also can make/receive a telephone call.
Recently, it has become fundamental in many countries to provide a phone call service to hearing-impaired people based on the TTY/TDD service, and a TTY/TDD function has become popular in mobile communication standards. Thus, the TTY/TDD service has been provided by wireless terminals.
As well known, 13K Qualcomm Code Excited Linear Prediction (QCELP) and Enhanced Variable Rate Code (EVRC) vocoders are representative voice compression techniques widely used in 2 G and 3 G mobile communication systems. In these vocoders, the TTY/TDD function is implemented as IS-733-3 and IS-127-4, respectively. That is, the TTY/TDD function of the EVRC vocoder is defined in IS-127-4 which is incorporated herein by reference, and the TTY/TDD function of the QCELP vocoder is defined in IS-733-3 which is also incorporated herein by reference.
The encoder of such a vocoder processes voice samples in 20-ms frame basis and transmits the processed voice information. The decoder of the vocoder receives voice information and reproduces voice samples in a reverse order to the encoder operation. The encoder in which the TTY/TDD function is implemented determines whether input Pulse Code Modulation (PCM) samples are voice or TTY/TDD text. In case of voice, the encoder transmits voice information, and in case of TTY/TDD text, the encoder transmits text information. The decoder also determines whether received information is voice information or text information by analyzing the received information and reproduces voice samples or TTY text tones according to the result of the determination. The TTY/TDD function supports voice carryover/hearing carryover (VCO/HCO). The VCO allows TTY/TDD text receiving and speaking to be switched, and the HCO allows TTY/TDD text transmitting and hearing to be switched. A communication system equipped with the encoder and decoder of the vocoder in which the TTY/TDD function is implemented enables communications between a hearing-impaired person and a person without any speech or hearing disabilities and a hearing-impaired person or between hearing-impaired persons.
However, since current wireless terminals provide none of a call connection request signal, a call ready signal, and an absence signal during a TTY/TDD service, hearing-impaired people cannot immediately recognize whether a call is in a connection state or in an absence state.